The background of the invention is discussed in relationship to one specific application of the invention—sizing of furnaces in residential and small commercial buildings. The present invention is not, however, limited to this specific application. This specific application is merely one example of the larger problem of properly sizing HVAC equipment. Many residential and small commercial buildings contain oversized furnaces and boilers. Due to low profit margins, many contractors merely guess the size of needed heating equipment. In the residential market, 85% of heating equipment is grossly (double correct size) oversized, 5% is undersized and only 10% is close to the designed heat loss of the structure. In contrast, the budget for heating, ventilation and air-conditioning (HVAC) in large buildings allows for more accurate studies of the heat loss and gain of the structure. Therefore, correctly sized equipment is usually found in these larger projects.
One significant, but generally overlooked, problem in residential furnace installations is oversized furnaces installed in undersized duct systems. An oversized furnace has many problems. In particular an oversized furnace will have a lower efficiency, require more maintenance, have a shorter lifetime and provide less comfort to an occupant.
An oversized furnace will cycle on and off quickly. The oversized furnace is able to blow hot air into the space being heated very rapidly in order to increase the temperature to the temperature level set by the thermostat. Most (oversized) furnaces installed in homes today are only on for 4 to 8 minutes at a time. In contrast, a correctly sized furnace will have burner on times of 15 to 16 minutes. This difference in burner on times is significant.
Rapid burner cycling dramatically shortens the life of the heat exchanger in the furnace, usually by 20 to 50 percent. Every time the burners switch on, the heat exchanger expands. Every time the burners switch off, the heat exchanger contracts. This constant expansion and contraction cycling can damage the heat exchanger over time. Because of the rapid cycling of an oversized furnace, the life of the heat exchanger is lessened.
This detrimental flexing of the heat exchanger metal is further exacerbated if the duct system is too small or restricted and the limit switch of the furnace adds yet another on and off cycle. Heat exchanger failures occur on equipment as new as 4–6 years old, although the heat exchangers were intended and expected to remain operational for a significantly longer period of time.
The rapid cycling of oversized equipment also negatively affects the chimney/vent. The furnace is not on long enough to raise the chimney temperature above the vaporization temperature of the flue products. The condensate produced by low chimney temperatures is acidic and can quickly destroy the vent. Even some grades of stainless steel fail when exposed to this condensation. Thus, improper furnace size creates significant problems.
Installing an oversized furnace may also result in a mismatch with the existing duct system. A new furnace generally has a lower temperature rise rating than the replaced furnace. In other words, the new furnace needs a larger volume of air forced through the duct system if the output BTUH (British thermal unit per hour) rating is the same. Some new furnaces actually need double the CFM (cubic feet per minute) as the replaced furnace. Very few duct systems will allow these dramatically increased air flows to happen. Adding a big blower may provide the necessary air volumes, but at the expense of high noise and lower comfort levels. A properly sized new furnace, with its lower air requirements, is more likely to fit with the existing duct system.
An oversized furnace costs more in installation and equipment costs than a properly sized smaller furnace. Furthermore, a properly sized furnace increases comfort levels throughout the home because the heat is provided at a lower, slower level. The occupants are unaware of the furnace cycles. This higher level of comfort often allows occupants to lower the thermostat a degree or two without any problems, saving more energy.
Problems with oversizing of furnaces are particularly noticeable with two-speed or variable speed furnaces. The primary advantages of variable speed furnaces include smooth, quiet and longer heat cycles, when operating on the low input setting. These furnaces still having the capacity to operate at a high stage if colder temperatures demand it. When operating at low speed, a variable speed furnace can provide longer heating cycles which allow for more even floor to ceiling temperatures, less room-to-room temperature difference and therefore more comfort to the home occupants. These advantages of two-speed furnaces are not achieved if the furnace is sized incorrectly such that the high stage of the furnace is never actually initiated, and the cycles of the low stage are too short.
The HVAC industry has attempted to solve this problem. The industry standard for sizing a furnace to be installed in a residence is Manual J, “Residential Load Calculation,” by the Air Conditioning Contractors of America, herein incorporated by reference in its entirety. This methodology can take the form of a computer program that does the heat loss calculations using built in formulas, or it can be done manually with a long form that requires the contractor to do the calculations. Both options require the contractor to take measurements of the windows, doors and exposed surface areas of the home. One significant problem with this approach is the lack of knowledge of just how much insulation is in the wall, ceiling or floor. In addition, the rate of infiltration allowed by the building envelope is unknown. Manual J has some excess built-in to make up for these unknowns, resulting in a larger furnace than necessary. In new homes, where insulation and infiltration amounts are known, Manual J still errs on the large side. Manual J is not a load calculation. At best, it is a load estimate. Further, contractors are discouraged from using Manual J due to the number of measurements that must be taken and the complexity of the calculations that must be made. Therefore problems remain.
U.S. Pat. No. 4,621,528 to Alt et al discloses a monitor apparatus and method of determining appliance size. In Alt, a total time and a total furnace on time are recorded. This information is combined with outside temperature, inside temperature, and an outside design temperature to calculate appliance size. One of the problems with Alt relates to accuracy. In particular, instead of looking at on/off cycling, only total on times and total off times are examined and average temperatures are used. Due to temperature fluctuations over the course of an evening and fluctuations in furnace on time, this methodology is inaccurate. In addition, Alt requires all complex calculations to be performed by hand which can be laborious and introduces the opportunity for errors. Further, Alt requires that outdoor temperature either be manually monitored or obtained after the fact from the National Weather Service which is time consuming and not necessarily accurate if the outdoor temperature is taken at a location with a different temperature than immediately outside of the structure. Furthermore, the Alt method is simply impractical for a contractor, HVAC service person, or other professional to perform for an occupied home in a manner that would ensure that they received accurate data. Either the professional would need to be present late at night and early in the morning or else the professional would need to rely upon the occupants to start the device and stop the device at the proper time. This requires the professional to rely too heavily on the occupants. Alt does not provide for ensuring the capture of an entire cycle. Also, in Alt, inaccuracies would also result if the house temperature was higher than the thermostat temperature when the AH clock began. Also, Alt would be particularly inaccurate at high temperatures. Therefore significant problems remain.
In summary, there is a need to provide an accurate and less-time consuming method and system for sizing HVAC equipment.
It is therefore a primary object, feature, or advantage of the present invention to improve over the state of the art.
Another object, feature, or advantage of this invention is to provide a method and system of measuring the actual heat loss of a home/structure.
A further object, feature, or advantage of the present invention is to provide a method and system for properly selecting a furnace to fit a home/structure.
A still further object, feature, or advantage of the present invention is to increase comfort levels of occupants of a home/structure.
Another object, feature, or advantage of the present invention is to save energy.
Yet another object, feature, or advantage of the present invention is to save money in HVAC equipment purchase and maintenance costs.
A further object, feature, or advantage of the present invention is to provide a method and system for sizing HVAC equipment that can be used to identify false cycling.
A still further object, feature, or advantage of the present invention is to provide a method and system for sizing HVAC equipment that is convenient enough and sufficiently reliable and accurate that it can be used for regulation and code enforcement of HVAC installations.
Another object, feature, or advantage of this invention is to provide a method and system of measuring the actual heat loss of a home/structure such that the effect of improvements such as installation of insulation, replacement windows and doors can be quantified.
At least one of these and/or other objects, features and/or advantages of the present invention will become apparent from the specification and claims that follow.